Frontend support for Nios2 target.

Summary:
- Implements TargetInfo class for Nios2 target.
- Enables handling of -march and -mcpu options for Nios2 target.
- Definition of Nios2 builtin functions.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D33356

Author: belickim <mateusz.belicki@intel.com>
llvm-svn: 304994